I worked in one "unicorn" once and all I did was getting protobufs from one API, putting it to database and taking it back from database and putting it to different API; it was so boring. I solved my boredom by contributing to some in-house framework, after which I was told I am out of my line and should go back to copying data between APIs.
Are people at Facebook actually solving these hard CS problems in daily life? Why are these interviews even a thing?